A night vision system converts available low intensity ambient light to a visible image. These systems require some residual light, such as moon or star light, in which to operate. This light is generally rich in infrared radiation, which is invisible to the human eye. The ambient light is intensified by the night vision system to produce an output image which is visible to the human eye. The present generation of night vision systems use image intensification technology and, in particular, microchannel plates to amplify the low level of visible light and to render a visible image from the normally invisible infrared radiation. The image intensification process involves conversion of the received ambient light into electronic patterns and the subsequent projection of the electron patterns onto a receptor to produce an image visible to the eye. Typically, the receptor is a phosphorous screen which is viewed through a lens provided as an eyepiece.
Specific examples of microchannel plate amplification are found in the image intensifier tubes of the night vision devices commonly used by police departments and by the military for night time surveillance, and for weapon aiming. However, microchannel plates may also be used to produce an electric signal indicative of the light flux or intensity falling on a photocathode, and even upon particular parts of the photocathode. The resulting electrical signals can be used to drive a video display, for example, or be fed to a computer for processing of the information present in the electrical analog of the image.
In night vision devices, a photoelectrically responsive photocathode element is used to receive photons from a low-level image. Typically the low-level image is far too dim to view with unaided natural vision, or may only be illuminated by invisible infrared radiation. Radiation at such wavelengths is rich in the night-time sky. The photocathode produces a pattern of electrons (hereinafter referred to as "photoelectrons") which correspond with the pattern of photons from the low-level image. Through the use of electromagnetic fields, photoelectrons emitted from the photocathode are directed to the surface of a microchannel plate.
The pattern of photoelectrons is then introduced into a multitude of small channels (or microchannels) opening onto the surface of the plate which, by the secondary emission of electrons, produce a shower of electrons in a pattern corresponding to the low-level image. That is, the microchannel plate emits from its microchannels a proportional number of secondary emission electrons. These secondary emission electrons form an electron shower thereby amplifying the electrons produced by the photocathode in response to the initial low level image. The shower of electrons, at an intensity much above that produced by the photocathode, is then directed onto a phosphorescent screen. The phosphors of the screen produce a visible image in yellow-green light which replicates the low-level image. Understandably, because of the microchannel plate, the representative image is pixalized, or is a mosaic of the low-level image.
More particularly, the microchannel plate itself conventionally is formed from a bundle of very small cylindrical tubes which have been fused together into a parallel orientation. The bundle is then sliced to form the microchannel plate. These small cylindrical tubes of the bundle thus have their length arranged generally along the thickness of the microchannel plate. That is, the thickness of the bundle slice or plate is not very great in comparison to its size or lateral extent; however, the microchannels individually are very small so that their length along the thickness of the microchannel plate is still many times their diameter. Thus, a microchannel plate has the appearance of a thin plate with parallel opposite surfaces.
The microchannel plate may contain over a million microscopic tubes or channels communicating between the faces of the microchannel plate. Each tube forms a passageway or channel opening at its opposite ends on the opposite faces of the plate. Further, each tube is slightly angulated with respect to a perpendicular from the parallel opposite faces of the plate so that electrons approaching the plate perpendicularly can not simply pass through one of the many microchannels without interacting with the interior surfaces.
Internally the many channels of a microchannel plate are each defined by or are coated with a material having a high propensity to emit secondary electrons when an electron falls on the surface of the material. In addition, the opposite faces of the microchannel plate are conventionally provided with a conductive metallic electrode coating so that a high electrostatic potential can be applied across the plate. As previously indicated, an electrostatic potential is also applied between the photocathode and the microchannel plate to move the photoelectrons emitted by the photocathode to the microchannel plate. Consequently, electrons produced by the photocathode in response to photons from an external image travel to the microchannel plate in an electron pattern corresponding to the received pattern of low-level light. These electrons enter the channels of the microchannel plate and strike the angulated walls which are formed by the secondary electron emissive material. Thus, the secondary emission electrons in numbers proportional to the number of photoelectrons exit the channels of the microchannel plate to impinge on a phosphorescent screen. An electrostatic field between the microchannel plate and the phosphor screen drives the electrons to the screen producing an intensified mosaic image of the low-level scene.
A conventional microchannel plate is known in accord with U.S. Pat. No. 4,737,013, issued 12 Apr. 1988, to Richard E. Wilcox. This particular microchannel plate has an improved ratio of total end open area of the microchannels to the area of the plate. As a result, the photoelectrons are not as likely to miss one of the microchannels and impact on the surface of the microchannel plate to be bounced into another one of the microchannels. Such bounced photoelectrons, which then produce a number of secondary electrons from a part of the microchannel plate not aligned with the proper location of the photoelectron, provide noise or visual distortion in the image produced by the image intensifier. The image intensifier taught by the Wilcox patent solves this problem of the conventional technology.
Understandably, because the microchannel plate is supplying a considerable number of electrons which become a part of the electron shower on the phosphorescent screen, the plate is subject to an electrical current between the metallic electrodes on its opposite faces. This electrical current between the opposite faces of a microchannel plate is known as a "strip current" and a portion of this current "replenishes" the channel wall with electrons lost by the emission process. This strip current is a function of the electrical resistance of a microchannel plate.
Conventional image tubes and photomultiplier tubes are also known which make use of cascaded microchannel plates. That is, multiple microchannel plates are arranged in series so that the initial electrons from a photocathode, for example, fall into the first microchannel plate. From this first microchannel plate, the secondary electrons from the first plate fall into a second microchannel plate. This second microchannel plate adds its own secondary emission electrons, and provides an increasingly intense shower of electrons. This shower of electrons may flow to a third or subsequent microchannel plate for further multiplication. In this way a very high electron gain or amplification may be effected, with each initial electron falling into the first plate resulting in several hundred to several hundred thousand electrons flowing from the last microchannel plate of the cascade. This electron shower may flow to a phosphor screen for producing a visible image, or to a solid plate acting as the anode to collect charge. At the anode, the electron shower becomes a current in a conductor which may be processed to count initial electrons or, in the case of multiple electrodes, to generate an image electronically, for example.
With the conventional photomultiplier tubes using cascaded microchannel plates, the electrostatic potential is connected across the top electrode of the top microchannel plate and the bottom electrode of the last or bottom microchannel plate in the cascade, with reference to the direction of electron flow. Although the microchannel plates of such a conventional photomultiplier tube are in facial contact with one another and are electrically connected in series, they are not otherwise connected. Each of the microchannel plates in the cascade experiences the same strip current flow.
However, conventional technology using cascaded microchannel plates all suffer from the deficiency that some of the individual microchannel plates in a cascade of microchannel plates are generally not fully secured in the tube housing. While one or more of the cascaded microchannel plates may be secured to the housing of the tube, others of the cascaded microchannel plates are secured in position simply by their facial frictional contact and interface with adjacent microchannel plates in the cascade. While this construction of the conventional devices has been satisfactory for most of the image intensifier tubes and photomultiplier tubes of the conventional technology, greater uses for these devices have revealed use environments in which the unconstrained microchannel plates may be vibrated or jarred out of position. That is, if an image intensifier tube, for example, is subjected to an impact or jarring which shifts one or more of the cascade of microchannel plates relative to the other microchannel plates of the cascade, then the optimal operating relationship of these cascaded microchannel plates may be disturbed. The result may be a degradation of the image quality provided by the image tube or total failure of the microchannel plate cascade. The same applies with respect to photomultiplier tubes using cascaded microchannel plates.
Regardless of how many microchannel plates are used in an image intensifying apparatus, the amplified signal from the plates may be detected and displayed using a number of different techniques depending on the needs of the user. For example, rather than directing the electron shower from a microchannel plate to a phosphorescent screen to produce a visible image, the shower of electrons may be directed upon an anode in order to produce an electrical signal indicative of the light or other radiation flux incident on the photocathode. If a single anode is disposed at the location ordinarily occupied by the phosphorescent screen, this anode will provide a current indicative of the photons received from a low-level scene. If the single anode is pixelated into a grid or array of anodes, the various anode portions of the grid or array will provide individual electrical signals which are an electrical analog of the image mosaic. Consequently, these electrical signals can be used to drive a video display, for example, or be fed to a computer for processing of the information present in the electrical analog of the image. Such an array may be used as multichannel particle detectors, providing information regarding the occurrence of particle collisions or other events and presenting the data in the form of computer-generated video and graphical representations.
Alternatively, such a microchannel plate can be used as a "gain block" in a device having a free-space flow of electrons. That is, the microchannel plate provides a spatial output pattern of electrons which replicates an input pattern, and at a considerably higher electron density than the input pattern. Such a device is useful as a particle counter to detect high energy particle interactions which produce electrons.
No matter which display format is used, an electrostatic field must be maintained between the display electrode or anode and the microchannel plates. As previously discussed, it is this relatively strong field which accelerates the multiplied electrons from the plates to the display electrode where they impact the selected collector and are converted to provide the desired information. Conventional image intensifiers typically employ a non-conductive ring or stand-off placed between the output surface of the plate and the electrode to maintain the proper separation. However, in the intense electrical potential needed to accelerate the multiplied electrons to the display electrode, conventional stand-offs often fail and allow current to flow over adjacent surfaces. This, in turn, disrupts the electrostatic field between the microchannel plate and the display electrode destroying the image and rendering the apparatus inoperable.
Accordingly, it is an object of the present invention to provide an apparatus which secures cascaded microchannel plates in place preventing their radial dislocation.
It is another object of the present invention to provide photomultiplier tubes and image intensifier tubes incorporating securely fixed cascaded microchannel plates to prevent radial dislocation relative to the tube and to one another.
It is still another object of the present invention to provide particle amplification tubes having an improved capacity to resist disruptive electrical shorts in internally established electrostatic fields.